Absorption Spectrum of CoCl2.6H2O PDF

Experiment3: Spectrophotometric Analysis Absorption Spectrum of Cobalt (II) Chloride Mrs. Nahid Chehade 1  Theory ▪ Spectrophotometric analysis is based on the use of the attenuation of electromagnetic radiation. ▪ It passe...

Experiment3: Spectrophotometric Analysis Absorption Spectrum of Cobalt (II) Chloride Mrs. Nahid Chehade 1  Theory ▪ Spectrophotometric analysis is based on the use of the attenuation of electromagnetic radiation. ▪ It passes through a sample (expressed in Transmittance or Absorbance) to quantitatively analyze a solution, thus determine its concentration. 2  Electromagnetic radiation Is a form of energy that has a wave nature. Therefore we can describe it by referring to its wavelength or frequency. It also has a particle’s nature and its energy is expressed by E=hν=hc/λ 3  Spectrophotometer Absorbance is measured with an instrument called a spectrophotometer. This instrument separates light into its component wavelengths and selectively measures the intensity of the light of a given wavelength after it passes through a solution. All spectrophotometers, regardless of the manufacturer, have certain common fundamental parts. These parts include a source of radiant energy, a prism or grating to isolate the light of a particular wavelength, a device for holding the sample, and a photoelectric cell for measuring the intensity of the light. 4 Spectrophotometer Spectro - photometer 5  Transmittance Is the ratio of the electromagnetic power existing in the sample to that incident on the sample from the source of radiation. T=Pt/P0 6  Absorbance Is the more common unit of expressing the attenuation of electromagnetic radiation and is defined as: A= -logT= -log Pt/P0= log P0/Pt 7  Beer Lambert Law According to Beer Lambert law, the absorbance is a linear function of the analyte’s concentration. Therefore, absorbance measurements are used to determine the concentration of a sample through which an electromagnetic radiation is passed. A: absorbance of the sample (no unit.) a(ε): analyte absorptivity (cm-1 conc-1) or extinction coefficient. If the concentration unit is M-1, then a is the molar absorptivity expressed in cm-1 M-1. b: pathlength of sample cell (cm). C: molar concentration of the analyte (mole/L) or (M). “a” is constant for a given substance at a particular wavelength. Its value may be zero if no light is absorbed. 8  Beer’s Lambert Law ▪ Absorbance is proportional to the concentration according to Beer’s Law As a solution is diluted, the Absorbance of the solution is decreased Beer lambert’s law only applies to dilute solutions. Therefore, a dilution of the solution is often necessary before the analysis is performed. 9  Blank Is used to correct the result (absorbance measured) obtained as several phenomena besides the absorbance of the analyte, contribute to the final reading. (A blank have zero absorbance). A BLANK IS COMPOSED OF ALL COMPONENTS OF THE SAMPLE MATRIX EXCEPT FOR THE ANALYTE ITSELF In this experiment: Distilled water is the blank 10  Absorption Spectrum and λmax ▪ When the analyte is the major absorbing species in the matrix, an absorption spectrum is realized. ▪ One wavelength “maximum wavelength” λmax is selected at which the subsequent measurements take place. Absorption spectrum: A graph of absorbance (A) versus wavelength (λ) is plotted to determine (λ max) Maximum wavelength (λ max): It is a characteristic value of a solution It is the wavelength at which the absorbance is maximum It is used in the construction of a standard curve Note: (λ max) doesn’t change with dilution. Ex: solution of CoCl2.6H2O 0.1M and 0.5M ,the 2 solutions have the same λ max = 500nm. 11  Calibration curve or standard curve ▪ When a graph of absorbance “A” versus concentration “C” is plotted for the standard solution, a linear relation should result which is the calibration curve. 12  Procedure - Mark each of 5 test tubes with one of a series of identification numbers running from 1 to 5. - Use pipets to make the additions of 0.150M CoCl2 and distilled water into the flasks as shown in the table. - Close the flasks with their stoppers and thoroughly mix their contents. Do not use one of your fingers as a stopper! - Turn on the spectrophotometer and leave it for 15 minutes to warm up. Set the wavelength at the value of the determined λmax. Blank preparation - Make a blank by filling a clean cuvette with distilled water Wipe the cuvette with tissue to clean fingerprints or content droplets. Hold the cuvette on the rough sides! - Insert the blank cuvette into the spectrophotometer and set the absorbance to 0 or transmittance to 100. Analysis of dilute solutions - Remove the blank cuvette and measure the absorbance of the different prepared solutions (1 to 5) Record your results in the table. Analysis of an unknown solution of CoCl2 - Measure the absorbance of the unknown solution (given by the instructor) following the same procedure previously described and record it. 13 A-Determination of maximum wavelength (λmax) λ(nm) A Absorption spectrum: Absorbance 400 0.063 versus λ 420 0.116 0.8 440 0.194 Absorbance 0.6 460 0.39 0.4 480 0.503 0.2 500 0.644 0 0 200 400 600 510 0.623 λ(nm) 520 0.577 540 0.33 λmax=500nm corresponding to the highest 560 0.182 absorbance A=0.644 14 Solutions were prepared by diluting a known volume from the Stock solution that was 0.15M in concentration. Absorbance readings were made at λmax=500nm. Tube# ml of ml of distilled Absorbance Concentration CoCl2.6H2 water O (C0V0=CV) 1 1 4 0.156 0.15x1=C1x5 →C1=0.15x1/5= 0.03M 2 2 3 0.321 0.15x2=C2x5 →C2=0.15x2/5= 0.06M 3 3 2 0.489 0.15x3=C3x5 →C3=0.15x3/5= 0.09M 4 4 1 0.645 0.15x4=C4x5 →C4=0.15x4/5= 0.12M 5 5 0 0.818 0.15x5=C5x5→ C5=0.15x5/5= 0.15M 15 B-The following standard graph was drawn based on the collected data. Calibration curve: Absorbance versus Concentration 0.9 y = 5.4152x 0.8 R² = 0.9999 0.7 Absorbance 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Concentration a)Determine Beer’s constant K: Beer’s Law: A= abC = KC Slope = K = (A3-A1)/(C3-C1)=(0.489- 0.156)/(0.09-0.03)= 5.55M-1 b)If the path length of the sample cell is 1cm: b=1 cm. Calculate the molar absorptivity of CoCl2.6H2O K= ab →a = K/b = 5.55/1 = 5.55 M-1cm-1 16 c) The absorption of an unknown solution of CoCl2.6H2O is 0.456. Determine the concentration of this unknown in 2 ways: 1) Theoretically: Beer’s law: A=abC C= A/ab = A/K =0.456/5.55= 0.0821M. 2) Graphically: On the standard curve, we put the value of absorbance A=0.456 on (y-axis) then we trace downward to see which concentration matches up to it. 17